Even the most robust exercise program won't grant you true fitness if you don't take control of your diet.

Exercise changes virtually every tissue in the body via many different pathways — metabolic, hormonal, neurological and mechanical — says Jennifer Sacheck, a nutritional biochemist and exercise physiologist at the John Hancock Research Center on Physical Activity, Nutrition and Obesity Prevention at the Friedman School of Nutrition Science and Policy at Tufts University. To supply these changes, athletes need to consume specific nutrients. Junk food supplies little or nothing; the right food choices, says Sacheck, allow your body to make the most of the time and effort you're dedicating to exercise and help you achieve your fitness goals faster.

At the Sports Performance Center I founded at the University of California-Davis, registered dietician and health nutritionist Marlia Braun notices that many athletes try to take control of what they eat by eliminating a food group or skipping meals altogether.

"It's difficult to make good nutritional choices, do food prep or eat moderately with this strategy," says Braun. She warns athletes against avoiding food groups, severely restricting calories, skipping meals or ignoring hunger, and recommends eating smaller amounts of good food with greater frequency instead.

Carbohydrates

Carbohydrates are essential for improving your fitness level, says Braun. Carbohydrates form glycogen, which is stored in your liver and muscles to provide energy during exercise. When your glycogen stores are low, you hit the wall. Braun notes that people on low-carb diets often can't sustain their intensity or endurance. Breads, potatoes and rice contain carbs, but so do grains, fruits, vegetables, beans and dairy, and these also contain an indigestible form of carbohydrate called fiber, which athletes need to the tune of about 25 to 30 grams a day to feel fuller between meals.

Protein

Your muscles are made up of protein, so as you build lean body mass through exercise, it makes sense that you need protein. Your body actually has not one but many proteins, and each serves a different function. When you eat protein, you break it down into its basic units: amino acids. Braun explains that you then build the specific proteins you need from those amino acids. You need 20 amino acids to construct the various proteins your body requires to function. Your body can build 11 of them; nine, however, it cannot, so you need to eat them, in animal products (beef, chicken, turkey, fish, eggs and pork) or soy (tofu, soy milk, miso). Beans and grains are missing one or more of those nine amino acids. In fact, beans are missing one amino acid and grains are missing another. If you combine these two protein sources, however, you get all of the essential amino acids you need.

Protein (along with fiber) helps you feel awake and bridges the gap between meals. More, however, is not better. Protein supplements don't build more muscle, as the makers of some bodybuilding powders claim. To the contrary, eating more protein than your body requires each day can affect your kidneys and liver (which excrete the nitrogen by-product of protein) and store what's left as fat. Braun recommends that athletes choose a lean protein to eat at every meal.

Fats

Fats, too, are important to athletes, she notes. Your muscles burn fat during endurance activities, so some healthier fat is necessary for fuel. These include polyunsaturated fats and omega-3 and omega-6 fats from nuts, vegetable oils, flaxseed and fish. Avoid fats found in animal products, palm oil, coconut oil, and trans-fat or partially hydrogenated oil. But don't avoid all fats.

You will improve your fitness by consuming good fats in the right amount, about 30 percent of your total calories, notes Braun. That's about 3 grams of fat or 27 calories from fat for every 100-calorie serving.